There are hoisting machines, such as electric chain blocks and electric hoists, which use an electric motor having a pull-rotor brake as a hoisting motor. The electric motor having a pull-rotor brake is configured as follows (detailed later). When the coil of the motor stator is not energized, the brake is activated, and the motor shaft is placed in a state of being constrained (braked). When the coil of the motor stator is energized, the brake is released by the action of a magnetic flux generated from the motor stator and that of the pull rotor. Thus, the motor shaft becomes unconstrained, and the motor rotor rotates.
As has been stated above, the electric motor with a pull-rotor brake has the advantage that the brake can be released to operate the electric motor simply by supplying an electric current to the coil of the motor stator. It is, however, necessary to supply the motor stator with sufficient electric current to release the brake when the electric motor is to be started. In the case of a variable-speed hoisting machine that is soft-started by using an inverter, the motor stator is not supplied with sufficient electric current to release the brake instantaneously when the electric motor is to be started. Therefore, there are problems such as that the brake cannot be released, or that the electric motor is started and operated with the brake dragging, for example, and the service life is reduced by overheating of the brake.
As a measure to solve the above-described problems, it is conceivable to apply the technique of an inverter-driven variable-speed hoisting machine described in Patent Literature 1. The electric motor of the variable-speed hoisting machine does not have a pull-rotor brake but operates as follows. At the start of a lifting operation, the inverter is operated according to a predetermined voltage-frequency (V-F) pattern, as shown by the dotted line in FIG. 1, from a state where the voltage is at a predetermined level V0 and the frequency is 0. When the output frequency reaches a frequency f1, a predetermined overvoltage V3 is output to the electric motor and the brake as an output voltage, as shown by the solid line, until the output frequency reaches a frequency f2, thereby supplying the brake coil with an electric current generating sufficient attraction force to release the brake. After the output frequency has reached the frequency f2, the overvoltage V3 is canceled, and the voltage and the frequency are increased according to the predetermined voltage-frequency (V-F) pattern to perform an accelerating operation.
The above-described technique may be applied to a variable-speed hoisting machine equipped with an electric motor having a pull-rotor brake. That is, at the start of a lifting operation, the electric motor is supplied with the overvoltage V3 output from the inverter for a predetermined period of time, thereby energizing the electric motor with sufficient electric power to generate attraction force required to release the pull-rotor brake. This makes it possible to release the brake but suffers from the problem that, when the acceleration of the hoisting machine is large, the length of time required for the output frequency of the inverter to reach from f1 to f2 is short, so that electric power required to release the pull-rotor brake cannot be supplied to the electric motor. For example, the acceleration time required for the inverter output frequency to reach from f1=5 Hz to f2=8 Hz is short in the case of an electric chain block, i.e. 20 msec, as compared to that of an electric hoist, i.e. 40 msec, as shown below. Thus, at the start of operating an electric chain block, the length of time during which the overvoltage electric power is supplied to the electric motor from the inverter is short, so that the brake cannot be released.
[Electric Hoist]                Acceleration time: 0.8 sec (0→60 Hz)        Low-speed frequency: 10 Hz (frequency f3 in FIG. 6)        Overvoltage interval: 5 Hz→8 Hz (f1→f2 in FIG. 1)        Overvoltage (V4 in FIG. 2) application time: 40 msec        
[Electric Chain Block]                Acceleration time: 0.4 sec (0→60 Hz)        Low-speed frequency: 10 Hz (frequency f3 in FIG. 6)        Overvoltage interval: 5 Hz→8 Hz (f1→f2 in FIG. 1)        Overvoltage (V4 in FIG. 2) application time: 20 msec        
To solve the above-described problem that it is impossible to ensure a sufficient time for supplying an electric current required to release the pull-rotor brake, it is conceivable to adopt a method of ensuring an electric current for releasing the pull-rotor brake and maintaining the brake in the released state by reducing the overvoltage application start frequency (i.e. reducing the frequency f1 in FIG. 1) as in an inverter control apparatus disclosed in Patent Literature 2. However, reducing the output frequency f1 at the start of overvoltage application has an adverse effect on the power cycle of a switching device (IGBT) constituting the inverter (i.e. the service life of the switching device is reduced).